1. Field of the Invention
This invention relates to an apparatus for estimating tire air pressure in an automobile or the like and, more particularly, to realization of an apparatus capable of estimating tire air pressures indirectly with improved accuracy from vibrational components of tires during vehicle traveling.
2. Related Art
As a tire air pressure estimating apparatus, an apparatus described in Japanese Patent Application Laid-Open No. H.5-133831 and an apparatus described in Japanese Patent Application Laid-Open No. H. 6-328920 have been known.
In each apparatus, the apparatus extracts the vibrational components of wheel speed caused by vibrations of a tire from a wheel speed signal, finds a tire resonant frequency or a tire spring constant in the vertical direction of the tire, in the longitudinal direction of the tire, or in the torsional direction of the tire rotation, and estimates the air pressure in the tire from the found resonant frequency or the found tire spring constant.
In these tire air pressure-estimating apparatuses, tire air pressure can be estimated without requiring means for directly detecting the air pressure inside the tire such as a pressure sensor.
A technique for indirectly detecting tire air pressure takes notice of the fact that a certain relation holds between the air pressure in a tire and its rigidity. However, the tire rigidity depends on tire rubber hardness as well as on the air pressure in the interior space inside the tire. It is difficult to detect them separately. Also, the tire rubber hardness varies with temperature of the rubber.
In the past, the technique for indirectly detecting tire air pressure does not take account of variations of the tire temperature. Therefore, the rigidity of the tire varies as a whole for different tire temperature even if the air pressure in the interior space of the tire is the same. With the prior art indirect detection technique, since the variations of the tire temperature is not taken into account, the detected value of the tire air pressure differs from the actual value.
Consequently, when the prior art apparatus detects abnormally low tire air pressure and warns a driver of this abnormality, for example, various undesirable situations may take place. For instance, if the tire temperature rises to thereby lower the tire rubber hardness, although the actual tire air pressure is normal, the tire air pressure may be determined to be too low. In addition, even though the tire air pressure is too low, it may not be detected.
Accordingly, it is a first object of the present invention to provide an apparatus for estimating tire air pressure with improved accuracy by taking account of the tire temperature.
It is a second object of the present invention to provide an economical tire air pressure-estimating apparatus that, more preferably, has a device for producing a value associated with tire temperature. This device is disposed within a signal processor that performs arithmetical processing for estimating the tire air pressure. This arrangement dispenses with wires for introducing the tire temperature into the signal processor.
In the meantime, frequency components that are not associated with variations in the tire air pressure and thus create noises are contained in a signal containing the vibrational components of the tire. Therefore, when a resonant frequency of the vibrational components or a tire spring constant dependent on tire air pressure is extracted from the signal including the vibrational components of the tire, the signal is normally fed to a signal-processing filter to extract only signal components close to the resonant frequency dependent on the tire air pressure. This is referred to as a pretreatment.
Ideally, the frequency band of this signal-processing filter sufficiently covers a frequency range that contains the signal components corresponding to an air pressure range to be detected within the vibrational components dependent on the tire air pressure. The frequency range containing the signal components corresponding to the air pressure range to be detected does not simply refer to from high to low tire pressures in a used range. The range is varied by the tire temperature due to the nature of the rubber of the tire. At low temperature, the frequency to be detected is increased. Conversely, at high temperature, the frequency to be detected is lowered. Therefore, the frequency range containing signal components corresponding to the air pressure range to be detected is from a frequency under a low-temperature, high-pressure condition to a frequency under a high-temperature, low-pressure condition.
However, this frequency range contains many frequency components becoming noises. Therefore, if a filter that passes signals of a frequency range containing the signal components corresponding to the air pressure range to be detected is adopted, the performance of air pressure detection is made unstable. For this reason, use of a filter having a narrowed frequency passing band has been heretofore urged. Consequently, the accuracy of the estimated air pressure can be assured only in a limited temperature range or pressure range.
In view of these circumstances, a third object of the present invention is to provide a tire air pressure estimating apparatus capable of achieve both improvement in the accuracy of estimation of tire air pressure and system responsiveness by making the filtering characteristics variable.